Modern steppers for semiconductor lithography and micromachining have a depth of focus that is only a few microns. Since the vertical position of a wafer or other workpiece can easily move by more than that (because of irregularities in the wafer stage motion, irregularities in the wafer exposure surface such as bowing, and thermal expansion of the system components), it is known to measure the position of the wafer and, if necessary, change the wafer height or readjust the focus. Prior art systems have suffered from the limitation that the position sensors can not be allowed to block the beam, so that fixed position sensors must measure the height at a position considerably removed from the actual exposure location.
In the case of e-beam exposure systems, there are additional problems such as evaporation of the photoresist during exposure and subsequent contamination of the sensors and restrictions on the materials of the measurement system (non-magnetic and non-insulating) in order to avoid interference with the beam.